The Steroid Analytical Core will provide sophisticated analytical methodologies for measuring the androgen metabolome with sufficient sensitivity and selectivity in different biological matrices for all Projects in the Program, for the Administrative/Clinical/Biostatistics Core A and for the Biospecimen/Animal Model Facility Core B. These matrices include medium from prostate cancer lines maintained in culture, xenograft models of castrate resistant prostate cancer (CRPC), and serum from their immunodeficient murine hosts. Ultimately, these methods can be applied to patient specimens (serum, prostate and bone biopsies) to determine the efficacy of different androgen ablative therapies in CRPC, and to help identify mechanisms of resistance to these therapies. Stable-isotope dilution liquid chromatography-mass spectrometry (LC/MS) of androgens and their precursor pregnanes will be the methodology employed. The LC/MS assays developed and provided in this core will be unique since no existing assays with sufficient sensitivity and selectivity are currently available to detect and quantitate all the steroids of interest. A unique resource of the Steroid Analytical Core is access to a panel of recombinant ketosteroid reductases (aldo-keto reductases) that can be used as synthons to generate the required [13C]-labeled internal standards. The Steroid Analytical Core will provide access to state-of-the art instrumentation and methods that will be cost-effective to measure steroid levels (androgens and pregnanes) not routinely available in the laboratories of individual investigators. It will assist investigators in the design of their experiments so that they are compatible with the analytical tools available. The specific aims of the Core are as follows: [1] To provide stable isotope dilution LC/MS methodology for the analysis of ketoandrogens in a variety of biological matrices (e.g. serum, cell culture, prostate tissue, and prostate and bone biopsies); [2] To provide stable isotope dilution LC/MS methodology for the analysis of hydroxyandrogens in identical biological matrices; and [3] To provide stable isotope dilution LC/MS methodology for the analysis of pregnane precursors of androgens in identical biological matrices. Since these methods are not routinely available elsewhere the Steroid Analytical Core will have an impact on prostate cancer research beyond this P01.